The present invention relates to a thermal actuation device.
Such devices, also known as thermo-actuators or electro-thermal linear motors, usually comprise a housing in which a thermal head is located, i.e. a device comprising a body made from a heat-conductive material (e.g. metal), in contact with an electric heater. Said body contains a heat expandable material (such as wax), and, at least partially, a rod or thrust element; the electric heater typically consists of a PTC resistor with a positive temperature coefficient electrically supplied by means of two terminals.
With the supply terminals live, the electric heater generates heat causing a volume increase of the heat expandable material: such a volume change will cause a linear displacement of the thruster outside the head body to move an actuation shaft up to a predetermined position, generally set by a mechanical limit stop. Upon ceasing the power supply, the heater cools down and the heat expandable material will shrink, causing the shaft and thruster to go back to their initial rest position, eventually with the help of a recall elastic element, such as a spring.
Thermal actuators as above have a simple low-cost manufacture and are usually highly reliable; other further significant advantages consist of a considerable power the are able to develop compared to their small size, and above all their noiseless operation; for these reasons, thermal actuators or electro-thermal actuators are widely used in various fields, such as for household appliances and environmental air conditioning.
However, the above devices have a drawback in that the stroke length they are able to obtain for the actuation shaft is rather limited.
Typically, a standardized thermal actuator as above with an outer housing about 15xc3x9720xc3x9745 mm and a thermal head about 6xc3x976xc3x9720 mm, can move or actuate a charge of a few tenths of kilograms (e.g. 10-20 kg) for a displacement of a few millimeter (e.g. 6-8 mm).
In order to solve the drawback of a limited stroke, other devices have been recommended utilizing several thermal actuators.
For instance, EP-A-0 781 920 discloses an electro-thermal actuation device, in which the housings of two thermal actuators are solidly connected to another common container body; both thermal actuators, located in relevant fixed positions, are arranged in series to each other, for the relevant thrust elements to operate substantially along one same axis.
Both possible embodiments as described in the above document have the thrusters of the two thermal actuators either directed to opposite directions or facing each other; however, in both cases, said thrusters operate a thrust, on one side, to an anchoring means of the device, and on the other side to an actuation element, which will transmit the translation the device is provided for.
A plurality of different positions, i.e. a plurality of stable work positions for the actuation element can be obtained supplying one, the other or both thermal actuators operating in series; in particular, as of interest herein, a translation of the actuation element substantially equal to the sum of the useful strokes of the actuation shafts of both thermal actuators can be obtained by a simultaneous supply of both thermal actuators.
However, the device described in EP-A-0 781 920, which has a reliable manufacture and versatile utilization, is rather expensive and bulky; in this connection, another drawback concerning the device related to the document EP-A-0 781 920 is the presence of two functional elements being required (i.e. the anchoring means and the movable element), which extend from the two lengthwise ends of the main body of the device; the solution mentioned above also requires the use of at least two electric control elements.
It is the aim of the present invention to solve the above drawbacks.
In this frame, it is a first object of the present invention to provide a thermal actuation device, which is of simple compact manufacture, and while assuring both the reliability, power and noiseless features of common devices, is able to obtain significant strokes for a linearly movable actuation element, without requiring any complex or bulky mechanical kinematics, or any complex and expensive components and control circuits.
Another object of the present invention is to provide an actuation device comprising motion multiplying means, whose operating mode may be easily converted by orienting a component of said multiplying means in a different way, in particular making its movable actuation element capable of obtaining alternatively a thrust or a pull.
These and other aims, which will become apparent later, are obtained according to the present invention by an actuation device incorporating the features of the annexed claims, which are intended as an integral part of the present description.